As used herein a defibrillator refers to any device intended to revert (i.e., to eliminate or stop) a tachyarrhythmia and/or a fibrillation with electrical energy substantially exceeding the pacing energy provided by implantable cardiac pacemakers.
Implantable cardiac pacemakers generally protect against high-energy pulses from an external source by limiting the voltage across any electrode pair. When an external device applies such a high energy pulse to this electrode pair through a low impedance, high currents flow in the electrodes and the limiting device. These high currents may have a detrimental effect on patient tissue in the region of the electrodes.
Money U.S. Pat. No. 4,320,763 refers to a current-limiter device placed in series circuit between a pacemaker and the proximal end of an electrode, to prevent tissue damage adjacent the distal end of the electrode. Money discloses two circuits, one with field-effect transistors (FET) and one with a bipolar transistor, which limit and then hold the current at a fixed value of approximately 20 mA.
Langer U.S. Pat. No. 4,440,172 refers to circuitry intended to protect an ECG sensing amplifier against defibrillation pulses, including a high-value resistor in series with the amplifier input, and clamping diodes across that input, on the proximal side of the resistor. A protected pacing circuit, which couples high frequency energy via an isolating transformer to a demodulator, in turn applies the demodulated pulse to the pacing electrodes. A reverse-biased diode is said to protect this circuitry during defibrillation.
Leinders U.S. Pat. No. 4,595,009 refers to a defibrillator/pacemaker which connects together the pacing and sensing electrodes at the moment it delivers a defibrillating pulse.
Winstrom U.S. Pat. No. 4,745,923 refers to a protection circuit for an implantable device, connected in series with a lead of a pacemaker to protect against high voltages and currents of defibrillators and other sources. The disclosed circuit limits current in either direction in a single lead (the patent shows the circuit in the return, or positive, pacing lead), and uses foldback current limiting, which is frequently used in commercial electronic power supplies (See, Section 6.05, Horowitz and Hill, The Art of Electronics, Cambridge University Press, 1989). In a foldback current limiter, impedance remains low until current reaches a first preset limit. Once current exceeds this first limit, impedance remains high until current falls below a second preset limit that is lower than the first preset limit. The Winstrom limiter circuit has a low impedance path between its terminals consisting of a field effect transistor (FET) in series with a low-value resistor. A bipolar transistor opens the FET when the voltage across the low-value resistor exceeds the base-to-emitter threshold of the bipolar transistor (the first preset limit in the foldback current limiter).
Tarjan U.S. Pat. No. 4,787,389 refers to a defibrillator/pacemaker system which opens switches between the pacemaker and its leads during the time it activates its defibrillator.
Bocchi British patent 2,234,908 shows a defibrillator/pacemaker with a protection switch in series with the pacing tip electrode, and a control signal to open the switch during defibrillation and close the switch during pacing.
Pless U.S. Pat. No. 5,111,816 refers to a defibrillator/pacemaker with a single N-channel metal-oxide-semiconductor field-effect transistor (MOSFET) switch in series with each pacing and sensing lead. The device opens these switches when it delivers a shock. When opened, these switches do not allow current to enter the leads from positive voltage sources with respect to pacing ground. The shock generator has its negative power supply referred to pacing ground, so that it only produces positive voltages with respect to pacing ground, thus within the operating range of the protection circuit. In the shock circuit, a diode is in series with each shock delivery switch, to prevent reverse current from flowing in shock circuit switches when external high energy pulses arrive at the shock electrodes. The diodes are said to be added because semiconductor switches withstand high voltage applied with one polarity, but only a few volts applied with opposite polarity. This applies equally to MOSFETs, bipolar junction transistors, and insulated-gate bipolar transistors (IGBTS). Making the switches block current in both directions prevents current from flowing in the shock generator power supply leads during external high energy pulses.
It is apparent that an implantable electronic device, such as a defibrillator/pacemaker must protect its low energy leads, such as pacing and sensing leads, against defibrillation pulses and other high-energy pulses. Although the aforementioned references address this objective, each suffers from one or more deficiencies.
A problem with the Langer patent circuit is that the diode in series with one of the pacing electrodes protects against an external defibrillator only when that defibrillator output pulse happens to fall in a preferred direction upon Langer's circuit, namely the direction where the diode does not conduct. Langer's circuit also requires a transformer, which is difficult to implement with integrated circuit technology.
A problem with the Leinders patent circuit is that it short-circuits the pacing and sensing leads, allowing defibrillation current to flow in their electrodes, with possible adverse effects on patient tissue near these electrodes.
A problem with the Tarjan, Bocchi, and Pless patent circuits is that each opens the pacing and sensing leads, but only in response to a control signal from the implant when it delivers its own high energy pulse. None has any means to detect high energy pulses from external sources and activate the protection circuits automatically.
A problem with the Money patent circuit, which limits the current in the leads to a fixed value, automatically, is that limiting to a fixed value requires a limiting device to dissipate power equal to the product of the limiting current and the applied voltage. Applied power reaches at least 1000 V at 20 mA, or 20 W. The current in such circuits also tends to oscillate about the limiting value, considering the high applied power. These oscillations can adversely affect operation of nearby circuits. Moreover the circuit construction must permit dissipation of this power.
A problem with the Winstrom patent circuit, which provides an automatic foldback current limiter to overcome the oscillation and dissipation concerns of limitation at a constant value, is that the circuit only opens the MOSFET switch in a low-impedance path after first actuating a second bipolar transistor switch, whose delay can result in a high-current spike through the limiter when a defibrillator applies a pulse with fast risetime to the pacing and sensing leads. See Benson U.S. Pat. No. 4,823,796, which describes such an external defibrillator that produces a trapezoidal pulse by transistor discharge of a capacitor with no series inductor. Measurements show such circuits can produce slew rates of thousands of volts per microsecond. Any delay in the protection circuit will result in a current spike, which may couple to and affect nearby circuitry.
Another problem with the Winstrom patent circuit is that it refers only to a bidirectional limiter for use in a single lead. The purpose is to limit current in one lead in a lead pair when a defibrillator applies energy in either polarity. Consequently, in the dual-chamber defibrillator/pacemakers, with two lead pairs, the Winstrom circuit would need to be applied in three of the four leads and results in a relative high part count, i.e., more components than a unidirectional limiter located in each lead.
A problem with the Pless patent circuit is that the shock generator is referenced to ground, which requires positive control voltages for the shock generator and negative control voltages for pacing and sensing. This increases the circuit complexity.
There thus remains a need for improved circuits for protecting low energy pacing and Sensing leads and electrodes from high energy pulses.